1. Field of the Invention
The present invention relates generally to the fields of ceramics, powders, polymers and metallurgy. More specifically, the present invention relates to a thermally activated reversible suspension.
2. Description of the Related Art
Various types of suspensions that respond to environmental stimuli with a dramatic change in rheology have been described by the prior art, including (1) electro-rheological suspensions, (2) magneto-theological suspensions, (3) coagulated aqueous suspensions, (4) silica/syrup suspension.
In the 1940's, Willis M. Winslow reported the formation of interparticle chains within certain suspensions subjected to an electric field (U.S. Pat. No. 2,417,850, Mar. 25, 1947). The phenomenon Winslow described has several of the characteristics that have been observed for later suspensions. Winslow reported a suspension that, for instance, could be lifted from its container if held between charged electrodes, but was fluid in the absence of an electric field. The stiffening was said to be reversible "under action of shear". Electro-rheological fluids are said to be nearly Newtonian in the absence of an electric field, and to possess a yield point when electrified.
Nearly simultaneous to the discovery of the electro-rheological response was the observation of an analogous magneto-rheology. In magneto-rheological suspensions, the particles again link together in chains, induced by a magnetic rather than electric field. The properties of these fluids are apparently similar to those of electro-rheological suspensions.
A different type of system with interesting properties is the coagulated suspension of Al.sub.2 O.sub.3 in water. In this suspension, an interparticle network can be formed by the addition of salt at low pH resulting in a pasty, shapable structure. The particles are said to be loosely held to one another so that they rearrange to pack tightly together during forming operation. The properties have not yet been reproduced for other powders besides Al.sub.2 O.sub.3.
Another system reported to increase in viscosity with increasing temperature, is a suspension of fine silica particles in syrup. The viscosity of this suspension is said to increase with temperature because the adsorption of syrup molecules onto the silica
The increases, adding to the effective volume of the particles. The suspension was never Newtonian, even at cool temperatures (19.degree. C.) and very low solids content (7 vol %), so its behavior is not truly analogous to the other systems discussed above. No application was associated with this system.
The prior art is deficient in the lack of truly reversible fluid mixture of a polymer liquid and solid particles which is activated by temperature to change from a liquid-like state to a solid-like state and vice versa. The present invention fulfills this longstanding need and desire in the art.